A superduty fireclay refractory brick with 33 to 43 per cent by weight Al.sub.2 O.sub.3 and 1 to 3 per cent by weight alkali oxide for use as a large-format bottom block in a tin bath in the manufacture of flat glass is known from DE-OS 40 13 294. In that case, for improving the properties, in particular the alkali resistance, the gas permeability and the mechanical resistance, a brick is proposed which has an open porosity of 16 to 20 per cent by volume, a compressive strength of 35 to 60 N/mm.sup.2, an elasticity of compression modulus of 3000 to 10 000 N/mm.sup.2, a refractoriness under load (RUL) ta value of 1300.degree. to 1450.degree. C. and a maximum gas permeability of 4 nPm. This brick is based on the knowledge that a low gas permeability is essential for reduced alkali attack. The range of the proposed gas permeability is certainly taken as very wide. Despite this, there still remain doubts in the mind of the specialist as to whether a gas permeability of less than 4 nPm can be achieved with the characteristics stated in DE-OS 40 13 294. In this document the gas permeability is looked at in conjunction with the open porosity. With an open porosity of between 16 and 20 per cent by volume, a low gas permeability also becomes apparent. Raising the firing temperature also leads to a lowering of the porosity. On the other hand, a low porosity, in the order of magnitude of 16 per cent by volume, results in a brittle brick which is not very elastic. However, it seems doubtful whether this knowledge about the properties gained from small-format bricks can be applied to large-format bricks. A simple test size, which provides a guide to the alkali resistance, is demonstrated with the gas permeability or air permeability respectively. The RUL, also described as important, seems to be rather less significant. In contrast, nothing is mentioned about hydrogen permeability.
However, it is known that in such furnaces for the manufacture of flat glass according to the float technique, a tin bath is used, upon which the molten glass is transported. The furnace is closed from outside and placed under a reducing N.sub.2 atmosphere containing H.sub.2. Interactions take place between the glass bath, the tin bath and the superduty fireclay refractory bottom brick. Alkali from the glass precipitates at the surface of the superduty fireclay refractory bricks and soaks into these more or less deeply. A change in the mineral phases occurs, linked with differing coefficients of thermal expansion. Therefore, even with low temperature differences, layers of the superduty fireclay refractory bricks spall off which float up through the tin bath and contaminate the underside of the band of glass. On the other hand, hydrogen from the furnace atmosphere diffuses right through the superduty fireclay refractory brick and is conveyed upwards through the tin bath so that bubbles form on the underside of the band of glass. This transport also encourages the removal of alkali glass from the surface of the superduty fireclay refractory brick. All this interferes with the continuous manufacture of flat glass and leads to a reduction in quality. This leads to a concentration of tin in various valency stages on glass surface on the tin side. In the first instance here, the liquid tin reacts with the sulphate of the glass. The more hydrogen dissolved in the tin, the stronger this reaction.